Permeation instruments are used to measure the transmission rate of a target analyte, such as oxygen, carbon dioxide or water vapor, through a film of interest. Typical films subjected to permeation testing are polymeric packaging films such as those constructed from low density polyethylene (LDPE), high density polyethylene (HDPE), oriented polypropylene (OPP), polyethylene terepthalate (PET), polyvinylidene chrloride (PVTDC), etc. Typically, the film to be tested is positioned within a test chamber to sealing separate the chamber into first and second cells. The first cell (commonly referenced as the sensing cell) is flushed with an inert gas to remove any target analyte from the cell and the second cell (commonly referenced as the analyte cell) filled with a gas containing a known concentration of the target analyte. A sensor for the target analyte detects the presence of target analyte that has migrated into the first cell from the second cell through the film.
Permeation instruments typically employ a flow-through method or an accumulation method for sensing the presence of target analyte in the first cell. Briefly, the flow-through method uses an inert flushing gas to continuously pick up any target analyte that has migrated into the first cell and deliver it to a remote sensor. The accumulation method allows target analyte to build up in the first cell for an accumulation period, with the sensor either positioned within the first cell or the first cell flushed with a flushing gas after the accumulation period for delivery of accumulated target analyte to a remote sensor.
The flow through method allows virtually all sensor types to be used, but are expensive and complex systems. The accumulation method, while permitting the use of less sensitive inexpensive sensors to accurately measure permeation of a target analyte through a film even at very low transmission rates, suffers from significantly longer test times.
Coulometric sensors are sensors that follow Faraday's Law, and are generally preferred for use in permeation instruments as they provide a number of advantages, including (i) extreme accuracy, (ii) elimination of any need to calibrate, (iii) ultra-high sensitivity to analyte, (iv) high specificity for a single analyte, (v) lack of temperature sensitivity, (vi) lack of pressure sensitivity, (vii) minimal sensitivity to flow, and (viii) low cost.
Most coulometric sensors are electrochemical. Unfortunately, electrochemical sensors are susceptible to the rapid loss of electrolyte to the surrounding environment, resulting in a rapid decline in sensitivity and a short useful life. The traditional method to solve this problem is to limit environmental access to the electrolyte by covering the sensor with a selective membrane that allows essentially unrestricted passage of the target analyte while limiting the passage of water molecules, or permitting access to the electrolyte only through a limited number of capillary columns. While effective for reducing the loss of electrolyte and thereby increasing the useful life of the sensor, such covered sensors suffer from a 100 to 1000 times reduction in sensitivity and a concomitant loss of all the coulometric sensor benefits described above, with the single exception of low cost.
Such covered electrochemical sensors are not widely used with permeation instruments employing the flow-through method as they do not possess the necessary sensitivity, and are not widely used with permeation instruments employing the accumulation method because they consume some of the target analyte during the measurement process, requiring complex corrective calculations in an imperfect effort to “correct” the sensed data.
It is possible to design and construct a coulometric sensor that doesn't suffer from the rapid lose electrolyte or sensitivity, See, U.S. Pat. Nos. 4,973,395 and 5,053,116, but such sensors are prohibitively expensive for use in low-cost permeation testing instruments.
Accordingly, a substantial need exists for a permeation instrument that enjoys the benefits achievable by using a coulometric sensor without suffering from the limited useful life inherent with electrochemical sensors.